Self-Charging Power Cell converts Mechanical Energy into Stored Chemical Energy
One of the major bottlenecks in power generation for artificial muscles and implanted prosthetic devices is storing the power generated. If you are deriving energy from the body, you are usually deriving mechanical energy. This is converted into electricity to power the device, but there is no easy way to convert this into another form for long-term storage. So, if movement ceases and there is no secondary power source, the prosthetic or artificial muscle stops working. Whilst recharging the battery by micro-charge is possible, it is extremely inefficient especially considering the tiny trickle of charge involved.
What has been missing, is a way to convert mechanical energy into a chemical form that can then be stockpiled, and converted back to power as necessary. This key step means that in theory at least, the implant would have an indefinite lifespan. There would never be a need to operate on the patient to change the battery, as the battery would replenish itself over time.
A method of filling that gap has been derived by Georgia Tech researcher Zhong Lin Wang and his team. The team has developed a self-charging power cell that uses the trickle-charge from a piezoelectric membrane with the battery actually layered on either side of the membrane. This is the key behind making the whole thing work. In essence the piezoelectric sandwich becomes the battery itself.
People are accustomed to considering electrical generation and storage as two separate operations done in two separate units, said Zhong Lin Wang, a Regents professor in the School of Materials Science and Engineering at the Georgia Institute of Technology. We have put them together in a single hybrid unit to create a self-charging power cell, demonstrating a new technique for charge conversion and storage in one integrated unit.
The basic design of the cell is a cathode made from lithium-cobalt oxide and an anode consisting of titanium dioxide nanotubes grown atop a titanium film. These form the bread of the sandwich, with the piezoelectric film between them being made from polyvinylidene fluoride film, which is a compression-based piezoelectric material it generates a charge only when it is compressed rather than stretched. This property is key to the whole process.
When the cell is subject to mechanical compression when it is squeezed by surrounding muscles for example a small charge is produced which works in conjunction with the deformation caused by the compression, to turn the whole sandwich into a kind of chemical pump. Lithium ions are driven by the pump, from the cathode side to the anode side. The energy is then stored in the anode as lithium-titanium oxide. When the contraction ceases, and the material begins to expand ready for the next cycle, the film is no longer capable of generating electrical charge, and the pump shuts down, so ions cannot flow the other way.
When power is required, the electrons flow through the circuit and return to the cathode side via the circuit.
So far, the team have constructed and tested 500 separate variations on the base powercell design, with the intent of finding the best possible configuration for power generation.
Much of the mechanical energy applied to the cells is now consumed in deforming the stainless steel case the researchers are using to house their power cell. Wang believes the power storage could be boosted by using an improved case.
When we improve the packaging materials, we anticipate improving the overall efficiency, he said. The amount of energy actually going into the cell is relatively small at this stage because so much of it is consumed by the shell.
Beyond the efficiencies that come from directly converting mechanical energy to chemical energy, the power cell could also reduce weight and space required by separate generators and batteries.
One day we could have a power package ready to use that takes advantage of this hybrid approach, Wang said. Almost anything that involves mechanical action could provide the strain needed for charging. People walking could be generating electricity as they move.